1. Field of the Invention
This invention relates to the processing of semiconductor wafers to form integrated circuit structures thereon. More particularly, this invention relates to the use of a VHF/UHF plasma in the processing of semiconductor wafers.
2. Description of the Related Art
In the processing of semiconductor wafers to form integrated circuit structures thereon, plasma-assisted processes are often utilized either in deposition or etching process steps. In such processes, e.g., reactive ion etching (RIE), plasma etching, CVD facet, or conformal isotropic CVD, radio frequency power from a generator or power source is generally applied to electrodes within a vacuum processing chamber via a matching network of some kind which will maximize the power transfer from the generator or power source to the plasma. When an electric field of sufficient magnitude is established between electrodes in the vacuum chamber, the field accelerates electrons present in the gas which undergo collisions with gas molecules. Very little energy can be transferred through elastic processes, because of the large mass difference between electrons and atoms or molecules, so the electron gains energy from the field and eventually may collide inelastically with a gas molecule, exciting or ionizing it. Ionization can liberate additional electrons, which are, in turn, accelerated by the field. The process avalanches, causing gas breakdown, resulting in a steady state plasma when ionization and recombination processes are balanced. Highly reactive ions and radical species are produced which are used to etch or deposit materials on semiconductor wafers.
The power sources used for such prior art generation of plasmas conventionally utilized electromagnetic radiation at low frequencies ranging from about 10-400 kHz, at high frequencies ranging from about 13 to about 40 MHz (typically at 13.56 MHz), and at microwave frequencies ranging from about 900 MHz up to 2.5 GHz.
At low frequencies of 10-400 kHz, both ions and electrons can be accelerated by the oscillating electric field, as well as any steady state electric field or bias developed in the plasma, resulting in the risk of potential damage to sensitive devices being fabricated on the wafer. At higher frequencies of 13-40 MHz, the so-called high frequency range, steady state electrode sheath voltages may be developed ranging from several hundred to over 1000 volts. This creates a problem since device damage is a concern at voltages exceeding about 200 volts, depending upon the device structure, material, and other factors.
The problem of high sheath voltage has been ameliorated by the use of microwave power sources to excite the plasma, i.e., power sources at a frequency range of from about 900 to about 2.5 gHz. Such techniques produce a plasma at low particle energies, i.e., 10-30 ev. However, the use of microwave frequencies can result in loss of anisotropy (vertical nature) of the etch, apparently due to the lowering of the sheath voltage; and can slow the etch or deposition rate down, apparently due to lowering of the particle energy level. In fact, in some cases the threshold energy level for certain processes, e.g., reactive ion etching of SiO.sub.2, cannot be reached using only microwave energy to power the plasma.
Because of such shortcomings in the use of a purely microwave energy source, microwave energy has been used in combination with another power source at high frequency, e.g. 13.56 MHz, to raise the sheath voltage at the wafer sufficiently to achieve the desired anisotropy in the etch. Microwave ECR sources use a microwave source and a magnetic field such that the ECR condition is met, that is, the radian frequency of the microwave source w= B e/m where B is the magnetic field magnitude and e and m are electron charge and mass respectively. This produces a high density, low energy plasma at low pressures. A divergent magnetic field can be used to extract ions and accelerate them to higher energies, and/or a high frequency bias may be applied to the wafer to increase ion energy.
However, such usage and coordination of multiple power source systems further complicates the apparatus used in carrying out such etching and/or deposition processes. Furthermore, the use of an ECR system necessitates the use of lower operating pressures ranging, for example, from 0.1 to several milliTorr. This, in turn, results in a reduction in the maximum flow rate of etching or deposition gases through the reaction chamber unless a very large vacuum pump is used.
It would, therefore, be desirable to conduct plasma-assisted processes using a power source wherein sheath voltages could be developed low enough to avoid risk of damage to devices on the wafer, yet high enough to achieve desired anisotropy, and at reaction rates comparable to prior art processes.